On the average of every two to three years, but the real answer is that you should wash them when they are dirty and not before or long after.
A dirty rug will transfer a lot of dirt to a cloth, and the results of your testing will be unambiguous.
We do a better job than you can do and carpets clinic are better at dealing with color-run when that occurs.
We use giant tumblers to accomplish this.
Then, we professionally test colors for fastness before we wet a rug to determine how we will approach the job.
Fringe can be replaced, though, often, new fringe on an old rug looks inappropriate.
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